Thixomolding material, method for manufacturing thixomolding material, and thixomolded article

ABSTRACT

A thixomolding material includes: a metal body that contains Mg as a main component; and a coating portion that is adhered to a surface of the metal body via a binder and contains Si particles containing Si as a main component. An average particle diameter of the Si particles is 1 μm or more and 100 μm or less, and a mass fraction of the Si particles in a total mass of the metal body and the Si particles is 1.0 mass % or more and 30.0 mass % or less. The binder may contain waxes. A content of the binder may be 0.001 mass % or more and 0.200 mass % or less.

The present application is based on, and claims priority from JP Application Serial Number 2021-056781, filed Mar. 30, 2021, the disclosure of which is hereby incorporated by reference herein in its entirety.

BACKGROUND 1. Technical Field

The present disclosure relates to a thixomolding material, a method for manufacturing a thixomolding material, and a thixomolded article.

2. Related Art

Magnesium has properties such as a low specific gravity, a good electromagnetic wave shielding property, good vibration damping capability, good machinability, and good biosafety. Based on such a background, parts made of magnesium alloys are beginning to be used in products such as automobiles, aircraft, mobile phones, and notebook computers.

As a method for manufacturing a part made of magnesium, a thixomolding method is known. The thixomolding method is a molding method in which a pellet-like or chip-like material is heated in a cylinder to bring the material into a solid-liquid coexistence state in which a liquid phase and a solid phase coexist, thixotropy is developed by rotation of a screw, and the obtained semi-solidified product is injected into a mold. According to such a thixomolding method, since fluidity of the semi-solidified product is enhanced by heating and shearing, a part having a small thickness or a part having a complicated shape can be formed as compared with a die casting method.

For example, JP-T-2007-510545 discloses a metal-based composite material in which at least 2 vol % of a Mg₂Si phase is incorporated into a metal base material containing magnesium or a magnesium alloy, and the metal-based composite material is used for manufacturing a cast body by thixomolding. Specifically, magnesium granules or magnesium alloy granules, and silicon granules or silicon alloy granules are introduced to a thixomolding step and solidified under shearing. Accordingly, high-temperature characteristics of the cast body can be improved.

In the method described in JP-T-2007-510545, when the cast body is manufactured, magnesium granules and silicon granules are directly charged in the thixomolding step. Therefore, when a shearing force is applied by a screw in a heated cylinder, it is necessary to uniformly mix the granules. However, when the granules are mixed only using the screw, it is insufficient in separation and mixing time due to a difference in specific gravity of the granules, and thus the granules are not uniformly mixed with each other, and segregation of silicon or a silicon alloy is likely to occur in the mixture. When such segregation occurs, characteristics of the cast body cannot be sufficiently improved.

SUMMARY

A thixomolding material according to an application example of the present disclosure includes: a metal body that contains Mg as a main component; and a coating portion that is adhered to a surface of the metal body via a binder and contains Si particles containing Si as a main component. An average particle diameter of the Si particles is 1 μm or more and 100 μm or less, and a mass fraction of the Si particles in a total mass of the metal body and the Si particles is 1.0 mass % or more and 30.0 mass % or less.

A method for manufacturing a thixomolding material according to an application example of the present disclosure includes: a preparation step of preparing a mixture containing a metal body containing Mg as a main component, Si particles containing Si as a main component, a binder, and a solvent; a stirring step of stirring the mixture; and a debindering step of removing, by heating the stirred mixture, at least a part of the binder contained in the mixture to obtain a thixomolding material. A mass fraction of the Si particles in a total mass of the metal body and the Si particles is 1.0 mass % or more and 30.0 mass % or less, and a content of the binder in the thixomolding material is 0.001 mass % or more and 0.200 mass % or less.

A thixomolded article according to an application example of the present disclosure includes: a matrix portion that contains Mg as a main component; and a particle portion that is dispersed in the matrix portion and contains Mg₂Si as a main component. A maximum particle diameter of the particle portion is 1.0 μm or more and 50.0 μm or less, and when observing a cross section, |As−Ac|/Ac is 30.0% or less where As [%] is an area fraction of the particle portion in a range of 500 μm square starting from a surface, and Ac [%] is an area fraction of the particle portion in a range of 500 μm square centered on a point at a depth of 1 mm from the surface.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing an example of an injection molding machine used for a thixomolding method.

FIG. 2 is a cross-sectional view schematically showing a thixomolding material according to an embodiment.

FIG. 3 is a process diagram illustrating a method for manufacturing the thixomolding material according to the embodiment.

FIG. 4 is a partial cross-sectional view schematically showing the thixomolded article according to the embodiment.

FIG. 5 is an example of an X-ray diffraction pattern obtained for the thixomolded article according to the embodiment.

FIG. 6 is an observation image of a cut surface of the thixomolded article corresponding to Examples when observed with an optical microscope.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereinafter, a thixomolding material, a method for manufacturing a thixomolding material, and a thixomolded article according to the present disclosure will be described in detail based on embodiments illustrated in the accompanying drawings.

1. Thixomolding Method

First, a thixomolding method using a thixomolding material according to an embodiment will be described.

The thixomolding method is a molding method in which a pellet-like or chip-like material is heated in a cylinder to bring the material into a solid-liquid coexistence state in which a liquid phase and a solid phase coexist, then thixotropy is developed by rotation of a screw, and the obtained semi-solidified product is injected into a mold. According to such a thixomolding method, since fluidity of the semi-solidified product is enhanced by heating and shearing, a part having a small thickness or a part having a complicated shape can be formed as compared with, for example, a die casting method.

FIG. 1 is a cross-sectional view showing an example of an injection molding machine used for the thixomolding method.

As shown in FIG. 1, an injection molding machine 1 includes a mold 2, a hopper 5, a heating cylinder 7, a screw 8, and a nozzle 9. In the mold 2, a cavity Cv is formed. When a thixomolding material 10 is charged into the hopper 5, the thixomolding material 10 is supplied to the heating cylinder 7. The thixomolding material 10 supplied to the heating cylinder 7 is transferred while being heated by a heater 6 and being sheared by the screw 8. Accordingly, the thixomolding material 10 is semi-melted and slurried. The obtained slurry is injected into the cavity Cv in the mold 2 through the nozzle 9 without being exposed to the atmosphere. Then, the slurry injected into the cavity Cv is cooled to obtain a thixomolded article.

The hopper 5 may be charged with other materials together with the thixomolding material 10.

2. Thixomolding Material

Next, a thixomolding material according to an embodiment will be described.

FIG. 2 is a cross-sectional view schematically showing the thixomolding material according to the embodiment.

The thixomolding material 10 shown in FIG. 2 is a raw material to be used in the thixomolding method, and includes a chip-like metal body 11, a coating portion 12 that adheres to the surface of the metal body 11, and an adhesive portion 13 that contains a binder and adheres the metal body 11 and the coating portion 12.

2.1. Metal Body

The metal body 11 is, for example, a section obtained by machining or cutting an Mg-based alloy cast with a mold or the like. A method for manufacturing the metal body 11 is not limited thereto.

The metal body 11 contains Mg as a main component and contains various additive components. Examples of the additive components include lithium, beryllium, calcium, aluminum, silicon, manganese, iron, nickel, copper, zinc, strontium, yttrium, zirconium, silver, tin, gold, and rare earth elements, and a mixture of one or more of the additive components is used. Examples of the rare earth elements include cerium.

The main component refers to an element having the highest content in the metal body 11. The content of the main component is preferably more than 50 mass %, more preferably 70 mass % or more, and still more preferably 80 mass % or more.

The additive components may include aluminum and zinc. Accordingly, the melting point of the metal body 11 is lowered, and the fluidity of the slurry is improved. As a result, moldability of the thixomolding material 10 can be enhanced.

In addition, the additive components may include at least one selected from the group consisting of manganese, yttrium, strontium, and rare earth elements in addition to aluminum and zinc. Accordingly, mechanical properties, corrosion resistance, abrasion resistance, and thermal conductivity of the thixomolded article can be enhanced.

The additive components may be present in a form of a simple substance, an alloy, an oxide, an intermetallic compound, and the like in the metal body 11. In addition, the additive components may be segregated or uniformly dispersed in a crystal grain boundary of a metal structure such as Mg or an Mg alloy in the metal body 11.

An average particle diameter of the thixomolding material 10 is not particularly limited, and is preferably 0.5 mm or more, and more preferably 1.5 mm or more and 10 mm or less. By setting the average particle diameter within the above range, generation of bridges and the like in the heating cylinder 7 of the injection molding machine 1 can be prevented.

The average particle diameter of the thixomolding material 10 is an average value of diameters of circles having the same area as a projected area of the thixomolding material 10. The average value is calculated based on 100 or more thixomolding materials 10 selected at random.

An average aspect ratio of the thixomolding material 10 is preferably 5.0 or less, and more preferably 4.0 or less. In the thixomolding material 10 having such an average aspect ratio, a filling property in the heating cylinder 7 is enhanced and temperature uniformity during heating is improved. As a result, a thixomolded article having high mechanical properties and high dimensional accuracy can be obtained.

The average aspect ratio of the thixomolding material 10 is an average value of aspect ratios calculated based on major axis/minor axis in a projection image of the thixomolding material 10. The average value is calculated based on 100 or more thixomolding materials 10 selected at random. The major axis is the maximum length that can be taken in the projection image, and the minor axis is the maximum length in the direction orthogonal to the major axis.

2.2. Coating Portion

The coating portion 12 contains Si particles 14 containing Si as a main component. Specifically, for example, a plurality of Si particles 14 are adhered to the surface of the metal body 11 to form the coating portion 12.

The coating portion 12 preferably covers the entire surface of the metal body 11, or may cover a part of the surface.

The Si particles 14 are not particularly limited as long as they are particles containing silicon as a main component, and the Si particles 14 may be particles containing amorphous Si as a main component, or may be particles containing crystalline Si as a main component.

The average particle diameter of the Si particles 14 is 1 μm or more and 100 μm or less, preferably 1 μm or more and 25 μm or less, and more preferably 2 μm or more and 15 μm or less. By setting the average particle diameter of the Si particles 14 within the above range, the balance between a coverage of the coating portion 12 and the Si content in the thixomolding material 10 can be optimized. In addition, when the Si particles 14 are adhered to the surface of the metal body 11, the Si particles 14 are less likely to fall off.

When the average particle diameter of the Si particles 14 is less than the above lower limit value, the Si particles 14 are less likely to be dispersed, and thus the above-described balance may be deteriorated. On the other hand, when the average particle diameter of the Si particles 14 is more than the above upper limit value, the Si particles 14 may easily fall off.

In the thixomolding material 10, the mass fraction of the Si particles 14 in the total mass of the metal body 11 and the Si particles 14 is 1.0 mass % or more and 30.0 mass % or less, preferably 1.5 mass % or more and 25.0 mass % or less, and more preferably 5.0 mass % or more and 20.0 mass % or less. By setting the mass fraction of the Si particles 14 within the above range, it is possible to prevent a decrease in moldability of the thixomolding material 10 while preventing a large decrease in mechanical strength of the thixomolded article to be manufactured.

When the mass fraction of the Si particles 14 is less than the above lower limit value, the mechanical strength of the thixomolded article may not be sufficiently enhanced. On the other hand, when the mass fraction of the Si particles 14 is more than the above upper limit value, the moldability of the thixomolding material 10 may be deteriorated.

The coating portion 12 may contain a substance other than the Si particles 14. In this case, the content of the substance other than the Si particles 14 may be less than the content of the Si particles 14 in terms of mass ratio.

The Si particles 14 may contain an element other than Si. In this case, the content of the element other than Si may be less than the content of Si in terms of mass ratio.

2.3. Adhesive Portion

The adhesive portion 13 is interposed between the metal body 11 and the Si particles 14 or between the Si particles 14.

The adhesive portion 13 contains a binder. As the binder, organic materials that bond the metal body 11 to the coating portion 12 are used. Examples of the organic materials include various resins, waxes, alcohols, higher fatty acids, fatty acid metals, higher fatty acid esters, higher fatty acid amides, nonionic surfactants, and silicone-based lubricants. The various resins include: polyolefins such as polyethylene, polypropylene, and ethylene-vinyl acetate copolymers; acrylic resins such as polymethyl methacrylate and polybutyl methacrylate; styrene resins such as polystyrene; polyvinyl chloride; polyvinylidene chloride; polyamide; polyesters such as polyethylene terephthalate and polybutylene terephthalate; polyether; polyvinyl alcohol; polyvinyl pyrrolidone; or copolymers thereof. In addition, the binder may be a mixture containing at least one of these components and another component, or may be a mixture containing two or more of these components.

Among these, the binder preferably contains waxes, and more preferably contains paraffin wax or a derivative thereof. The waxes have a good binding property, and can strongly bond the metal body 11 to the Si particles 14 or strongly bond the Si particles 14 to each other. When using the waxes in combination with debindering conditions, it is possible to obtain a thixomolding material capable of reducing generation of gas during molding to a low level.

Examples of the waxes include natural waxes and synthetic waxes. The natural waxes include: plant waxes such as candelilla wax, carnauba wax, rice wax, Japan wax, and jojoba oil; animal waxes such as beeswax, lanolin, and spermaceti; mineral waxes such as Montan wax, ozokerite, and ceresin; and petroleum waxes such as paraffin wax, microcrystalline wax, and petrolatum. The synthetic waxes include: modified waxes such as synthetic hydrocarbons such as polyethylene wax, Montan wax derivatives, paraffin wax derivatives, and microcrystalline wax derivatives; hydrogenated waxes such as hardened castor oil and hardened castor oil derivatives; fatty acids such as 12-hydroxystearic acid; acid amides such as stearamide; and ester such as phthalic anhydride ester.

As described above, the thixomolding material 10 according to the embodiment includes the metal body 11 and the coating portion 12. The metal body 11 contains Mg as a main component. The coating portion 12 adheres to the surface of the metal body 11 via the binder, and contains the Si particles 14 containing Si as a main component. The average particle diameter of the Si particles 14 is 1 μm or more and 100 μm or less. The mass fraction of the Si particles 14 in the total mass of the metal body 11 and the Si particles 14 is 1.0 mass % or more and 30.0 mass % or less.

By performing thixomolding using such a thixomolding material 10, Si can be uniformly dispersed in a semi-molten state. Accordingly, in the thixomolding, primary crystals Mg₂Si are precipitated while being uniformly dispersed in the course of solidification. As a result, a thixomolded article having high rigidity can be obtained.

The thixomolding material 10 may contain additives other than the metal body 11, the coating portion 12, and the adhesive portion 13 described above. Examples of the additives include a coupling agent, a surfactant, a dispersant, a lubricant, an antioxidant, an ultraviolet absorber, a thickener, a rust inhibitor, a preservative, and a fungicide.

3. Method for Manufacturing Thixomolding Material

Next, a method for manufacturing the above-mentioned thixomolding material 10 will be described.

FIG. 3 is a process diagram illustrating the method for manufacturing the thixomolding material according to the embodiment.

The method for manufacturing the thixomolding material 10 shown in FIG. 3 includes a preparation step S102, a drying step S104, a stirring step S106, and a debindering step S108.

3.1. Preparation Step

In the preparation step S102, a mixture containing a metal body 11, Si particles 14, a binder, and a solvent is prepared. The metal body 11 is similar to the above-described metal body 11. In addition, the Si particles 14 are similar to the above-described Si particles 14.

The solvent is not particularly limited as long as it is a liquid in which the binder is dispersed. Examples of the solvent include water, isopropanol, and acetone. For mixing, a mixer, a kneader, or the like is used. This step may be a step of preparing a mixture prepared in advance.

The content of the binder in the mixture is not particularly limited, and is preferably 1.0 mass % or more and 30.0 mass % or less, more preferably 2.0 mass % or more and 15.0 mass % or less, and still more preferably 3.0 mass % or more and 10.0 mass % or less. By setting the content of the binder within the above range, the Si particles 14 can be uniformly dispersed based on a dispersion action of the binder.

When the content of the binder is less than the above lower limit value, an amount of the binder is insufficient, and it may be difficult to uniformly adhere the Si particles 14 to the metal body 11, and it may be difficult to uniformly disperse the Si particles 14. On the other hand, when the content of the binder is more than the above upper limit value, the amount of the binder becomes excessive, and the Si particles 14 which are not adhered to the metal body 11 may easily aggregate, or an amount of the binder residue increases in the debindering step S108 described later and a large amount of gas is generated in the heating cylinder, so that an internal defect may easily occur in the thixomolded article.

The temperature of the solvent is preferably set to be equal to or higher than the melting point of the binder, if necessary. Accordingly, the binder is easily dissolved in the solvent. As a result, the binder can be dispersed more uniformly. The temperature of the solvent is preferably set to be higher than the melting point of the binder by 10° C. or more, and more preferably set to be higher by 20° C. or more and 50° C. or less.

In this case, the above-described mixture may be placed in a container, and the entire container may be heated from the outside using a hot bath or the like.

The melting point of the binder to be used is not particularly limited, and is preferably 40° C. or higher and 80° C. or lower, more preferably 43° C. or higher and 65° C. or lower, and still more preferably 45° C. or higher and 60° C. or lower. When the melting point of the binder is within the above range, the binder can be efficiently melted in a short time. In addition, when the melting point of the binder is within the above range, the thixomolding material 10 to be manufactured may have good lubricity in thixomolding, and can increase melt fluidity of the slurry.

3.2. Drying Step

In the drying step S104, the mixture is dried. Accordingly, the Si particles 14 are adhered to the surface of the metal body 11 via the binder, and the solvent is volatilized to obtain a dried body. In the present embodiment, since the Si particles 14 are dispersed using the binder, the Si particles 14 can be adhered to the surface of the metal body 11 with a uniform thickness.

For drying, a method of heating the mixture, a method of exposing the mixture to a gas, or the like is used. Among these methods, when the mixture is heated, for example, the entire container containing the mixture may be heated using a hot bath or the like. In the drying step S104, the entire solvent in the mixture may be removed, or a part of the solvent may remain without being removed.

A temperature at which the mixture is heated may be equal to or higher than the temperature at which the solvent volatilizes and the binder softens, specifically, the temperature is set according to a composition of the solvent, and is preferably 40° C. or higher and 120° C. or lower, and more preferably 50° C. or higher and 80° C. or lower. Accordingly, the solvent can be volatilized and removed while preventing the Si particles 14 adhered to the surface of the metal body 11 from falling off.

In addition, a time for heating the mixture is appropriately set depending on the heating temperature, and is, for example, preferably 10 minutes or longer and 300 minutes or shorter, and more preferably 20 minutes or longer and 200 minutes or shorter.

The drying step S104 may be performed as necessary, and may be omitted, or the drying step S104 and the stirring step S106 may be performed at the same time.

3.3. Stirring Step

In the stirring step S106, the mixture is stirred. When the drying step is performed, the dried mixture is stirred. For stirring, a method using a stirring bar, a stirrer, or the like, a method of shaking a container containing a mixture with a lid, or the like is used. By such stirring, the Si particles 14 can be adhered to the surface of the metal body 11 via the binder. A part of the Si particles 14 may be directly adhered to the surface of the metal body 11 without interposing the binder. In addition, by stirring, formation of a block by aggregation of metal bodies 11 can be prevented.

After the stirring step S106, the drying step S104 and the stirring step S106 may be repeated as necessary. Accordingly, since the Si particles 14 are repeatedly adhered, the Si particles 14 can be adhered to the surface of the metal body 11 in multiple layers. As a result, more Si particles 14 can be adhered to the surface of the metal body 11. The number of repetitions is not particularly limited, and is, for example, 2 or more and 10 or less. Also in this case, the drying step S104 and the stirring step S106 may be performed at the same time.

3.4. Debindering Step

In the debindering step S108, a debindering treatment is performed on the stirred mixture. Accordingly, the thixomolding material 10 is obtained. Examples of the debindering treatment include a method of heating the mixture, and a method of exposing the mixture to a gas for decomposing the binder. Accordingly, at least a part of the binder contained in the mixture can be removed. As a result, by preventing a large amount of binder from being transferred into the heating cylinder 7, generation of a large amount of gas in the heating cylinder 7 can be prevented.

A heating temperature for the mixture in the debindering treatment is not particularly limited as long as it is a temperature at which the binder is thermally decomposed, and the heating temperature is preferably 200° C. or higher and 500° C. or lower, and more preferably 250° C. or higher and 450° C. or lower. By setting the heating temperature within the above range, the binder can be appropriately removed while preventing an adverse effect on the metal body 11 due to the debindering treatment.

When the heating temperature is lower than the above lower limit value, a large amount of binder which is not removed remains, and a large amount of gas may be generated in the heating cylinder 7. On the other hand, when the heating temperature is higher than the above upper limit value, there is a concern that an adverse effect due to heat may occur on the metal body 11, or the binder may be completely removed and the Si particles 14 may fall off from the metal body 11.

A heating time for the mixture in the debindering treatment is not particularly limited, and may be, for example, 5 minutes or longer, and is preferably 1 hour or longer and 100 hours or shorter, and more preferably 10 hours or longer and 50 hours or shorter. Accordingly, the binder can be appropriately removed while preventing an adverse effect on the metal body 11 due to the debindering treatment.

An amount of the binder, that is, a content of the binder in the thixomolding material 10 after debindering is not particularly limited, and is preferably 0.001 mass % or more and 0.200 mass % or less, more preferably 0.010 mass % or more and 0.100 mass % or less, and still more preferably 0.015 mass % or more and 0.040 mass % or less. By setting the content of the binder in the thixomolding material 10 within the above range, the amount of the binder to be thermally decomposed in the heating cylinder 7 can be prevented from increasing more than necessary while ensuring adhesiveness of the coating portion 12 realized by the adhesive portion 13.

When the content of the binder is less than the above lower limit value, the amount of the binder is insufficient, and the coating portion 12 may easily fall off. On the other hand, when the content of the binder is more than the above upper limit value, the amount of the binder becomes excessive, a large amount of decomposed gas is generated in the heating cylinder 7, and voids may be easily generated in the thixomolded article.

As described above, the method for manufacturing the thixomolding material 10 according to the present embodiment includes the preparation step S102, the stirring step S106, and the debindering step S108. In the preparation step S102, the mixture containing the metal body 11 containing Mg as a main component, the Si particles 14 containing Si as a main component, the binder, and the solvent is prepared. In the stirring step S106, the mixture is stirred. In the debindering step S108, the stirred mixture is heated to remove at least a part of the binder contained in the mixture, thereby obtaining the thixomolding material 10. The mass fraction of the Si particles 14 in the total mass of the metal body 11 and the Si particles 14 is 1.0 mass % or more and 30.0 mass % or less. The content of the binder in the thixomolding material 10 is 0.001 mass % or more and 0.200 mass % or less.

According to such a configuration, even when the amount of the Si particles 14 is large, the Si particles 14 can be adhered to the surface of the metal body 11 via the binder, and thus the Si particles 14 can be uniformly dispersed in the heating cylinder 7. Accordingly, a thixomolded article in which reaction chance between Mg and Si is uniformly ensured, and primary crystals Mg₂Si are uniformly dispersed and precipitated in the course of solidification can be manufactured. As a result, a thixomolded article having high rigidity can be obtained.

The thixomolding material 10 does not necessarily have to be manufactured by this manufacturing method. That is, the thixomolding material 10 may be manufactured, for example, without going through the debindering step S108.

4. Thixomolded Article

Next, a thixomolded article according to the embodiment will be described.

FIG. 4 is a partial cross-sectional view schematically showing the thixomolded article according to the embodiment.

The thixomolded article 100 shown in FIG. 4 is a molded article obtained by a thixomolding method, and includes a matrix portion 200 and a particle portion 300. The matrix portion 200 is a portion mainly derived from the metal body 11 of the thixomolding material 10, and contains Mg as a main component. The particle portion 300 is a portion mainly derived from a reaction product of the coating portion 12 of the thixomolding material 10 and Mg, and contains Mg₂Si as a main component.

As shown in FIG. 4, when viewing the cross section of the thixomolded article 100, an area occupied by the matrix portion 200 is larger than an area occupied by the particle portion 300. Therefore, the particle portion 300 is in a state of being dispersed in the matrix portion 200. The maximum particle diameter of the particle portion 300 is 1.0 μm or more and 50.0 μm or less.

FIG. 4 illustrates a range A1 of 500 μm square starting from a surface 101 of the thixomolded article 100, and a range A2 of 500 μm square centered on a point at a depth of 1 mm from the surface 101. When an area fraction of the particle portion 300 in the range A1 is defined as As [%] and an area fraction of the particle portion 300 in the range A2 is defined as Ac [%], |As−Ac|/Ac is 30.0% or less in the thixomolded article 100 according to the present embodiment.

In such a thixomolded article 100, a difference in occupied areas of the particle portion 300 between the range A1 located in the vicinity of the surface 101 and the range A2 located at a deeper position is reduced. That is, in the thixomolded article 100, uneven distribution of the particle portion 300 is prevented. The particle portion 300 is a portion containing Mg₂Si as a main component and having high rigidity. Therefore, such a thixomolded article 100 has high rigidity. The maximum particle diameter of the particle portion 300 is within the above range. Therefore, a decrease in mechanical strength due to the particle portion 300 being too large is also prevented.

The maximum particle diameter of the particle portion 300 is 1.0 μm or more and 50.0 μm or less, preferably 3.0 μm or more and 30.0 μm or less, and more preferably 5.0 μm or more and 20.0 μm or less.

In addition, |As−Ac|/Ac is 30.0% or less, preferably 25.0% or less, and more preferably 20.0% or less.

The area fraction As of the particle portion 300 in the range A1 is calculated as follows. First, in an observation image of the range A1, an area of the particle portion 300 is calculated by image processing. For the image processing, for example, image analysis software OLYMPUS Stream or the like can be used. A magnification of the observation image is preferably 300 times or more. Next, a ratio of the area of the particle portion 300 to a total area of the range A1 is calculated. This ratio is the area fraction As. The range A1 is a range having a square shape with a side of 500 μm, and at least a part of the range A1 may be in contact with the surface 101.

The area fraction Ac of the particle portion 300 in the range A2 is also calculated in the same manner as the area fraction As.

The range A2 is a range having a square shape with a side of 500 μm, and a center point O of the range A2 is a point at a depth of 1 mm from the surface 101. When a length in a depth direction is less than 2 mm in the cross section of the thixomolded article 100, a midpoint of the length in the depth direction can be regarded as the center point O.

The area fraction As and the area fraction Ac are determined by the content of Si in the thixomolded article 100, and are preferably 5% or more and 55% or less, more preferably 20% or more and 50% or less, and still more preferably 30% or more and 45% or less. Accordingly, the thixomolded article 100 has particularly high rigidity.

The maximum particle diameter of the particle portion 300 is calculated as follows. First, particle diameters of the particle portion 300 included in the range A1 and the range A2 are all measured. The particle diameter of the particle portion 300 is an intermediate value between a length of the major axis and a length of the minor axis in the observation image of the particle portion 300. The maximum value of the particle diameters calculated in this manner is the maximum particle diameter of the particle portion 300.

Further, the average particle diameter of the particle portion 300 is preferably 0.5 μm or more and 10.0 μm or less, and more preferably 1.0 μm or more and 5.0 μm or less. When the average particle diameter of the particle portion 300 is within the above range, the particle portion 300 has a small diameter as a whole, and thus the particle portion 300 is less likely to become a starting point of a crack or the like. Accordingly, in addition to the rigidity, mechanical strength such as bending strength and tensile strength of the thixomolded article 100 can be increased.

The average particle diameter of the particle portion 300 is, when all particle diameters of the particle portion 300 included in the range A1 and the range A2 are measured, an average value of the particle diameters of the particle portion 300.

The content of Si in the thixomolded article 100 is 1.0 mass % or more and 30.0 mass % or less, preferably 1.5 mass % or more and 25.0 mass % or less, and more preferably 5.0 mass % or more and 20.0 mass % or less. Accordingly, the thixomolded article 100 has particularly high rigidity.

Examples of measurement of the content of Si include: iron and steel-atomic absorption spectrometry defined in JIS G 1257:2000; iron and steel-ICP emission spectrometry defined in JIS G 1258:2007; iron and steel-spark discharge emission spectrometry defined in JIS G 1253:2002; iron and steel-X-ray fluorescence analysis defined in JIS G 1256:1997; and weight, titration, and absorption photometry defined in JIS G 1211 to G 1237.

An average aspect ratio of the particle portion 300 is preferably 3.0 or less, more preferably 2.5 or less, and still more preferably 2.0 or less. When the average aspect ratio of the particle portion 300 is within the above range, anisotropy of structures of the particle portion 300 is reduced. Therefore, the rigidity of the thixomolded article 100 can be isotropically enhanced.

The average aspect ratio of the particle portion 300 is calculated as follows. First, lengths of respective major axes and lengths of respective minor axes of the particle portion 300 included in the range A1 and the range A2 are respectively obtained. Next, a ratio of the lengths of the respective major axes to the lengths of the respective minor axes is referred to as an “aspect ratio”. An average value of the aspect ratios calculated in this manner is the average aspect ratio of the particle portion 300.

The tensile strength of the thixomolded article 100 is preferably 100 MPa or more and 350 MPa or less, and more preferably 150 MPa or more and 300 MPa or less. Further, the Young's modulus of the thixomolded article 100 is preferably 44 GPa or more and 80 GPa or less, and more preferably 50 GPa or more and 80 GPa or less.

The thixomolded article 100 having a tensile strength and a Young's modulus within the above ranges has particularly high specific strength and specific rigidity. Since such a thixomolded article 100 is lightweight and has high strength, and is thus suitable for, for example, parts used in a transportation device such as an automobile and an aircraft, parts used in a mobile device such as a mobile terminal and a notebook computer, and the like.

The tensile strength of the thixomolded article 100 is measured as follows. First, a test piece is cut out from the thixomolded article 100. Examples of the test piece include a No. 13 test piece defined in JIS. Next, the test piece is attached to a tensile tester, and stress corresponding to the maximum force applied to the test piece at 25° C. is calculated. The obtained stress is defined as the tensile strength of the thixomolded article 100.

The Young's modulus of the thixomolded article 100 is measured as follows. First, a test piece is cut out from the thixomolded article 100. Next, the test piece is attached to a tensile tester, and a tensile load is applied to the test piece at 25° C. Next, an amount of change in tensile strain when the tensile load is varied and an amount of change in tensile stress when the tensile load is varied are respectively calculated. Then, a ratio of the latter amount of change to the former amount of change is calculated, and the obtained ratio is defined as the Young's modulus of the thixomolded article 100. The Young's modulus of the thixomolded article 100 may be a value measured by a method other than the above-mentioned measurement method, for example, a resonance method or an ultrasonic pulse method.

A Vickers hardness of the surface 101 of the thixomolded article 100 is preferably 80 or more and 350 or less, more preferably 90 or more and 300 or less, and still more preferably 100 or more and 250 or less.

When the Vickers hardness is within the above range, it is possible to obtain a thixomolded article 100 that has a high surface hardness and is less likely to be scratched.

The Vickers hardness of the surface 101 of the thixomolded article 100 is measured in accordance with a method of Vickers hardness test specified in JIS Z 2244:2009. A measurement load is 5 kgf.

FIG. 5 is an example of an X-ray diffraction pattern obtained for the thixomolded article according to the embodiment.

As shown in FIG. 5, when the thixomolded article 100 is subjected to a crystal structure analysis by an X-ray diffraction method, an X-ray diffraction pattern including a peak derived from α-Mg, a peak derived from β-Mg₁₇Al₁₂, and a peak derived from Mg₂Si is obtained. The α-Mg is a matrix phase of the matrix portion 200 and is a solid solution mainly containing Mg.

When a CuKα ray is used as the X-ray, the main peak derived from α-Mg is observed at 2θ=36.5° to 37.5°, and the main peak derived from Mg₂Si is observed at 2θ=39.5° to 40.5°. The main peak derived from α-Mg refers to a peak having the maximum peak intensity among peaks derived from α-Mg. The main peak derived from Mg₂Si refers to a peak having the maximum peak intensity among peaks derived from Mg₂Si.

When the peak intensity of the main peak derived from α-Mg is taken as 100, the peak intensity of the main peak derived from Mg₂Si is preferably 20 or more and 250 or less, and more preferably 50 or more and 200 or less. In the thixomolded article 100 having such a peak intensity ratio, α-Mg and Mg₂Si are present in a well-balanced manner, so that both high rigidity and high strength are achieved. Therefore, a thixomolded article 100 having particularly excellent mechanical properties can be obtained.

The thixomolding material, the method for manufacturing the thixomolding material, and the thixomolded article according to the present disclosure are described above based on the illustrated embodiments. However, the thixomolding material and the thixomolded article according to the present disclosure are not limited to the above embodiment, and may be, for example, those obtained by adding any component to the above embodiment. The method for manufacturing the thixomolding material according to the present disclosure may be one obtained by adding any desired step to the above embodiment.

Examples

Next, specific examples of the present disclosure will be described.

5. Manufacturing of Thixomolding Material 5.1. Sample No. 1

First, a magnesium alloy chip as a metal body, Si particles, a binder, and a solvent were mixed to obtain a mixture. As the magnesium alloy chip, a chip of 4 mm×2 mm×1 mm made of an AZ91D alloy manufactured by STU, Inc. was used. The AZ91D alloy is an Mg-based alloy containing 9 mass % of Al and 1 mass % of Zn. In addition, as the binder, “Paraffin Wax 115” manufactured by Nippon Seiro Co., Ltd. was used. The melting point of the paraffin wax 115 was 48° C. Further, as the solvent, 35 mL of isopropanol was used per 4.5 g of the binder.

Next, the obtained mixture was heated to obtain a dried body. Subsequently, the obtained dried body was stirred. Thereafter, an operation of further heating the stirred dried body and then stirring the heated dried body was repeated three times. For stirring, a method of shaking a container containing the dried body was used.

Next, the stirred dried body was subjected to a debindering treatment. Accordingly, at least a part of the binder was removed to obtain a thixomolding material. In the obtained thixomolding material, almost the entire surface of the magnesium alloy chip was coated with the Si particles. Manufacturing conditions in the above manufacturing method are shown in Table 1. In Table 1, a charge amount of the Si particles is a ratio of a mass of the charged Si particles to a total mass of the magnesium alloy chip and the Si particles. A charge amount of the binder is a ratio of a mass of the charged binder to a mass of the entire thixomolding material.

5.2. Sample Nos. 2 to 5

Thixomolding materials were obtained in the same manner as in Sample No. 1 except that the manufacturing conditions were changed as shown in Table 1.

5.3. Sample No. 6

A thixomolding material was obtained in the same manner as in Sample No. 1 except that the Si particles and the binder were omitted and accordingly the manufacturing conditions were changed.

5.4. Sample Nos. 7 to 14

Thixomolding materials were obtained in the same manner as in Sample No. 1 except that the manufacturing conditions were changed as shown in Table 1.

5.5. Sample No. 15

A thixomolding material was obtained in the same manner as in Sample No. 1 except that the Si particles were used, but the binder was omitted, and accordingly the manufacturing conditions were changed.

In Table 1, among the thixomolding materials of the respective sample Nos., those corresponding to the present disclosure were referred to as “Examples,” and those not corresponding to the present disclosure were referred to as “Comparative Examples”.

6. Evaluation of Thixomolding Material

6.1. Amount of Si Particles after Debindering

For the thixomolding material of each sample No., an amount of the Si particles after debindering is calculated by the following method.

First, a mass M1 of the thixomolding material was measured. Since the thixomolding material is debindered, the remaining binder is regarded as substantially zero, and is not considered for calculation. Next, the thixomolding material was immersed in acetone and washed with an ultrasonic cleaner for 10 minutes. Accordingly, the adhered Si particles can be removed, and only the magnesium alloy chip can be taken out. Next, the magnesium alloy chip after washing was taken out from acetone, dried, and then a mass M2 was measured.

Then, a mass fraction of the Si particles with respect to the magnesium alloy chip calculated by (M1−M2)/M1×100 was defined as an amount [%] of the Si particles after debindering. Calculation results are shown in Table 1.

6.2. Adhesion Rate of Si Particles

An adhesion rate of the Si particles was calculated by dividing the amount of the Si particles after debindering by the charge amount of the Si particles. Calculation results are shown in Table 1. Amounts of the Si particles after debindering shown in Table 1 are values after an approximate calculation, whereas adhesion rates shown in Table 1 are values obtained by a calculation using values before the approximate calculation. Therefore, the adhesion rates shown in Table 1 may slightly deviate from values calculated based on charge amounts of the Si particles and the amounts of the Si particles after debindering shown in Table 1.

6.3. Amount of Binder after Debindering

For the thixomolding material of each sample No., an amount of the binder after debindering was calculated by the following method.

First, a thermogravimetric change in a temperature range of 50° C. to 450° C. of one thixomolding material was measured by a differential thermogravimetric simultaneous measurement device (TGA/DSC1LF) manufactured by Mettler-Toledo. The temperature was increased at a temperature increase rate of 10° C./min while air was allowed to flow in at a flow rate of 30 mL/min in the atmosphere. Then, in order to eliminate an influence of the solvent, a weight change at 450° C., with reference to a weight at 200° C., was calculated as the amount of the binder after debindering. Calculation results are shown in Table 1.

TABLE 1 Evaluation result of Manufacturing condition for thixomolding material thixomolding material Si particles Debind- Si particles Binder Average Binder Drying Number ering Debind- Amount Amount Example/ particle Charge Charge temp- Drying of temp- ering after Adhesion after Sample Comparative diameter amount amount erature time repetitions erature time debindering rate debindering No. Example μm mass % mass % ° C. min time(s) ° C. h mass % % mass % 1 Example 5 2.0 5 65 120 3 320 24 1.8 92 0.020 2 Example 10 8.0 8 65 120 3 400 24 7.8 97 0.018 3 Example 15 10.0 10 65 120 3 320 24 9.0 90 0.025 4 Example 15 18.0 10 65 120 3 320 24 15.2 84 0.029 5 Example 20 25.0 15 65 120 3 290 48 20.4 82 0.035 6 Comparative — 0.0 0 — — — — — — — — Example 7 Comparative 20 50.0 35 65 120 3 220 24 48.0 96 0.250 Example 8 Comparative 20 3.0 5 65 120 3 400 60 0.5 17 0.002 Example 9 Comparative 5 5.0 2.0 65 120 3 460 24 1.5 30 0.0005 Example 10 Example 2 10.0 5 65 120 3 320 24 7.3 73 0.021 11 Example 5 10.0 5 65 120 2 400 2 8.8 88 0.025 12 Example 50 10.0 10 65 120 0 250 12 8.0 80 0.035 13 Comparative 5 10.0 5 65 120 3 — — 4.5 45 4.9 Example 14 Comparative 5 10.0 5 65 120 1 220 1 7.0 70 0.250 Example 15 Comparative 5 10.0 0 — — — — — 1.0 10 — Example

As shown in Table 1, it is confirmed that in the thixomolding materials corresponding to Examples, although the amount of the binder is reduced to the minimum by debindering, the Si particles are adhered at a sufficient adhesion rate.

7. Manufacturing of Thixomolded Article 7.1. Sample No. 16

The thixomolding material of Sample No. 1 was charged into an injection molding machine to obtain a thixomolded article of Sample No. 16. As the injection molding machine, a magnesium injection molding machine JLM75MG manufactured by The Japan Steel Works, Ltd. was used.

7.2. Sample Nos. 17 to 27

Thixomolded articles were obtained in the same manner as in Sample No. 16 except that the manufacturing conditions were changed as shown in Table 2.

8. Analysis of Thixomolded Article 8.1. Cross Section Observation

The thixomolded article of each sample No. was cut, and the cut surface was observed with an optical microscope. FIG. 6 is an observation image when the cut surface of the thixomolded article corresponding to Examples is observed with the optical microscope. In FIG. 6, a particle portion exhibiting a dark color and a matrix portion exhibiting a light color are observed. Most of the particle portion has a relatively small aspect ratio and an isotropic shape. The obtained average aspect ratios and maximum particle diameters of the particle portion are shown in Table 2.

In addition, ranges A1 and A2 as shown in FIG. 4 were identified, and the area fractions As and Ac of the particle portion were calculated. Then, |As−Ac|/Ac were calculated in terms of percentage. Calculation results are shown in Table 2.

8.2. Content of Si

A content of Si in the thixomolded article of each sample No. was measured by elemental analysis. Measurement results are shown in Table 2.

9. Evaluation of Thixomolded Article 9.1. Moldability

The thixomolded article of each sample No. was observed, and a molded state of the thixomolded article was evaluated based on melt fluidity, presence or absence of internal defects due to inclusion of blowholes and air, and the like. Specifically, those having many defects in melt fluidity and internal defects were evaluated as “NG,” and those having relatively few such defects were evaluated as “OK”. Evaluation results are shown in Table 2.

9.2. Tensile Strength

The tensile strength of the thixomolded article of each sample No. was measured. Specifically, a test piece conforming to JIS standard was formed from the thixomolded article, and the tensile strength was measured by a tensile tester. Measurement results are shown in Table 2.

9.3. Young's Modulus

The Young's modulus of the thixomolded article of each sample No. was measured. Measurement results are shown in Table 2.

9.4. Vickers Hardness

The Vickers hardness of the surface of the thixomolded article of each sample No. was measured. Measurement results are shown in Table 2.

TABLE 2 Manufacturing condition for thixomolded article and analysis result Particle portion Average Average Evaluation result of thixomolded article Example/ Sample aspect particle |As-Ac|/ Content Tensile Young's Vickers Sample Comparative No. of ratio diameter Ac of Si Moldability strength modulus hardness No. Example material — μm % mass % — MPa GPa — 16 Example 1 1.5 12.8 15.8 1.6 OK 221 44 102 17 Example 2 1.8 13.4 14.5 7.0 OK 178 47 111 18 Example 3 2.0 14.6 12.3 8.5 OK 158 50 111 19 Example 4 2.2 15.8 10.3 14.3 OK 101 61 151 20 Example 5 2.5 19.6 16.8 19.2 OK 130 72 214 21 Comparative 6 — — — 0.0 OK 192 42 73 Example 22 Comparative 7 2.0 22.5 52.6 42.0 NG 85 55 190 Example 23 Comparative 8 2.2 0.8 21.0 0.3 OK 193 42 73 Example 24 Comparative 9 10.0 6.5 35.9 0.5 OK 180 41 74 Example 25 Example 10 1.5 4.8 12.6 6.0 OK 170 45 109 26 Example 11 2.3 11.6 9.7 5.0 OK 160 44 108 27 Example 12 2.8 32.6 22.6 7.2 OK 165 46 110

As is clear from Table 2, it is confirmed that the thixomolded articles corresponding to Examples have higher rigidity than the thixomolded articles corresponding to Comparative Examples. In addition, it is confirmed that when the content of Si is too low, the rigidity cannot be sufficiently enhanced, and on the other hand, when the content of Si is too high, the moldability is poor.

Further, in Comparative Examples in which no binder is added in the manufacture of the thixomolding material, the rigidity of the thixomolded article cannot be enhanced. The reason for the above includes that Si particles fall off from the magnesium alloy chip and the Si particles cannot be sufficiently dispersed. 

What is claimed is:
 1. A thixomolding material comprising: a metal body that contains Mg as a main component; and a coating portion that is adhered to a surface of the metal body via a binder and contains Si particles containing Si as a main component, wherein an average particle diameter of the Si particles is 1 μm or more and 100 μm or less, and a mass fraction of the Si particles in a total mass of the metal body and the Si particles is 1.0 mass % or more and 30.0 mass % or less.
 2. The thixomolding material according to claim 1, wherein the binder contains waxes.
 3. The thixomolding material according to claim 1, wherein a content of the binder is 0.001 mass % or more and 0.200 mass % or less.
 4. A method for manufacturing a thixomolding material, comprising: a preparation step of preparing a mixture containing a metal body containing Mg as a main component, Si particles containing Si as a main component, a binder, and a solvent; a stirring step of stirring the mixture; and a debindering step of removing, by heating the stirred mixture, at least a part of the binder contained in the mixture to obtain a thixomolding material, wherein a mass fraction of the Si particles in a total mass of the metal body and the Si particles is 1.0 mass % or more and 30.0 mass % or less, and a content of the binder in the thixomolding material is 0.001 mass % or more and 0.200 mass % or less.
 5. A thixomolded article comprising: a matrix portion that contains Mg as a main component; and a particle portion that is dispersed in the matrix portion and contains Mg₂Si as a main component, wherein a maximum particle diameter of the particle portion is 1.0 μm or more and 50.0 μm or less, and when observing a cross section, |As−Ac|/Ac is 30.0% or less where As [%] is an area fraction of the particle portion in a range of 500 μm square starting from a surface, and Ac [%] is an area fraction of the particle portion in a range of 500 μm square centered on a point at a depth of 1 mm from the surface.
 6. The thixomolded article according to claim 5, wherein an average aspect ratio of the particle portion is 3.0 or less.
 7. The thixomolded article according to claim 5, wherein a tensile strength is 100 MPa or more and 350 MPa or less, and a Young's modulus is 44 GPa or more and 80 GPa or less. 